Field of the Invention
This invention relates to optical systems for use with flight vehicles subject to extreme aero-thermal heating in which window self-emission reduces the signal-to-noise ratio (SNR) of sensed target emissions complicating the task of target tracking.
Description of the Related Art
Optical systems for use with flight vehicles such as guided missiles or belly-mounted sensor pods on aircraft typically include an optical window (e.g., a dome) that protects the sensitive optical and electrical components. The optical window is transparent in a desired spectral band (e.g., the MWIR band from 3-5 microns) to pass emissions from a target in a scene through the optical window to the entrance pupil of focusing optics, which in turn route the incident radiation along an optical path and focus the radiation onto a detector. The detector may, for example, be a quad-cell detector for non-imaging applications such as spot tracking. The detector may, for example, be a focal plane array (FPA) for various imaging applications. The FPA generally includes an array of pixels, each pixel including a photo-detector that generates a signal responsive to the intensity of the incident. These signals are collected and combined to form a digital image of the object. The focusing optics may be fixed or gimbaled to increase the field-of-regard (FOR). Typically, the entrance pupil is symmetric about the central axis of symmetry of the optical window. Alternately, the entrance pupil may be offset such that the FOR does not cross the tip of the optical window to reduce distortion (See U.S. Pub. No. 2015/0022874).
Ideally, the only emissions sensed by the detector are those from the scene and particularly a specified target. However, in guided missiles or sensor pods there can be many different sources of parasitic radiation or “noise” that reduces the SNR of the target and the ability of the guidance unit to track the target. One such source is the self-emission of the optical window that may occur due to aero-thermal heating as the missile or pod travels through the atmosphere. The amount of aero-thermal heating depends on the flight speeds, aerodynamic design of the window that induces heating and the thermal design of the window that removes heat to cool the window. The window self-emissions can raise the general background noise or can induce a gradient in the detected signal (due to non-uniform heating of the window). In many instances, the self-emissions due to aero-thermal heating are insignificant. In others, it is desirable to try to mitigate the effects of window self-emissions.
One approach is to spectrally filter the incident radiation. Generally speaking, the temperature of the aero-thermally heated optical window is much higher than the temperature of the target. As such, the emissions of the target and the optical window will have different spectral characteristics. For example, the relative intensity of the window emissions will be stronger at the longer wavelengths in the MWIR band. Low-pass filtering the incident radiation can improve the contrast of the target radiation (signal) to the window self-emitted radiation (noise). See, for example, U.S. Pat. No. 8,466,964 entitled “Multispectral Uncooled Thermal Infrared Camera System” issued Jun. 18, 2014.
Non-uniform aero-thermal heating can induce a gradient in the window self-emissions, hence the total detected incident radiation. This gradient is a form of “fixed-pattern noise”. One way to remove this fixed pattern noise is by using Scene based Non-Uniformity Correction (NUC), in which the scene is intentionally blurred and the resulting image is recorded and then subtracted from non-blurred images of the scene. The blurring is usually done by moving an optical element, such as a lens, prism, or diffuser, into the beam, though some scene-based NUCing methods are completely software-based. The advantage of scene based NUC (as opposed to other NUCing methods, such as the use of a shutter in front of the detector) is that is can correct for contributions to fixed-pattern noise from every optical element in the system, including windows and domes. The disadvantage is that the method of blurring must be carefully designed so that the target is not inadvertently subtracted from the final image. See E. E. Armstrong, M. M. Hayat, R. C. Hardie, S. Torres, and B. Yasuda, “Non-uniformity Correction for Improved Registration and High-Resolution Image Reconstruction in IR Imagery,” Proceedings of SPIE's Annual Meeting, Application of Digital Image Processing XXII, Denver Colo., Jul. 18-23, 1999.
Polarimeters can be used to analyze the polarization components of light to, for example, extract shape information from an object. Some polarimeters use two or more linear polarizers (polarized pixels) that filter at least half of the incoming light and direct the remaining light to a focal plane. As a result, the brightness of the image at the focal plane is substantially reduced (e.g., by about half).
Polarimetry requires at least three measurements to analyze the polarization components of light; at least two different polarization components and possibly an unpolarized component. Typically, the pixelated filter array, and FPA, are divided into groups of four pixels (e.g., a 2×2 sub-array of pixels). The standard commercially available pixelated filter array is a 2×2 array of linear polarizers having angular values of Θ1=0°, Θ2=45°, Θ3=90° and Θ4=135°, respectively, which are optimum assuming perfect alignment between the pixelated filter array and the FPA. U.S. Patent Publication 2014/0063299 to Fest et. al. entitled “Movable Pixelated Filter Array” describes a technique for using the data reduction matrix to account for misalignment. The electronics may compute an Angle of Linear Polarization (AoLP) image and a Degree of Linear Polarization (DoLP) image from the four linearly polarized pixel values in each grouping to extract shape information. The electronics may also compute an average of the four detector pixels in each grouping to form a reduced resolution intensity image.